Abstract: Salinity is an important ecological factor in the algal growth.Water loss would lead to the increase of salinity in mass culture of fish food diatom,which could affect the algal photosynthesis and inorganic carbon utilization.In this paper,impacts of salinity on the growth,carbonic anhydrase activity,photosynthesis(P-I curve)and chlorophyll a fluorescence parameters in Nitzschia closterium f. minutissima were investigated to explore the photosynthetic mechanism of the diatom under the high salinity.The results showed that the specific growth rate when Nitzschia closterium f. minutissima was grown in high salinity(70)was decreased by 59.2% compared with the normal seawater,but it can maintain the growth to a certain degree in salinity up to 60-70,which suggested that Nitzschia closterium f. minutissima had a stronger tolerance to high salinity.The extracellular carbonic anhydrase activity which was measured by an electrometric method was reduced by 66.3%,and Chl.a and Chl.c contents which was determined spectrophotometrically were also decreased by 50.0% and 45.7% in high salinity(70).The extracellular CA activity was inhibited in high salinity environment which showed that capacity of HCO^-₃ catalyzed by extracellular CA into CO₂ declined,and high salinity stress can damage the Chl.a/b light harvesting complex(LHCII)and reaction-center complex and affect pigment synthesis.The chlorophyll a fluorescence parameters were measured by saturation pulse,and the maximal efficiency of PSⅡ photochemistry(F_v/F_m),actual photochemical efficiency of PSⅡ(Yield),photochemical quenching co-efficient(qP)decreased,whereas non-photochemical quenching co-efficient(qN)increased when the algae were grown in high salinity.These results showed that the high salinity stress would damage the PSⅡ reaction center,inhibit primary reaction of photosynthesis as well as the process of photosynthetic electron transport.The photosynthetic rate in the varied light intensity and inorganic carbon concentration was determined with a Clark-type oxygen electrode,and maximal photosynthetic rate(P_max),photosynthetic efficiency(α)and the affinity for inorganic carbon(K_0.5)obtained by non-linear fitting from P-I and P-C curves also declined in high salinity.All the above results suggested that the growth and photosynthesis in Nitzschia closterium f.minutissima could be partially inhibited by higher salinity,but it also could adapt to high salinity stress through adjusting the activity of extracellular carbonic anhydrase,the affinity for inorganic carbon and photosystem Ⅱ energy flow and energy utilization efficiency.